Filters are optical elements with specific spectral transmission characteristics. They are used to modify the properties of a light source or to block unwanted wavelengths of light.
There are two different types of filters, depending on the physical principle exploited during the manufacturing process: absorption filters (colour filters) and dielectric filters.
Absorption filters (colour filters) function on the basis of choosing a glass recipe with chromophoric constituents which absorb a specific range of wavelengths. Typically these are long pass edge filters, but some bandpass types with limited functionality are available. Note that this filter type may fluoresce when used for blocking UV light.
Dielectric filters absorb very little light. The functionality is based on reflection of the unwanted spectral regions based on optical interference within the layer structure of the filter coating. These filters can be manufactured for use in the UV, visible and IR.
In order to reduce the number of coating layers – and thus the cost – a combination may be made by dielectric coating a colour (absorption type) glass substrate. This is common for some bandpass filters.
The following table gives an overview of the main criteria for deciding between these two physical principles of filters.
|Physical Principle||Price||Power Handling||Angle Tolerance||Thickness||Dysfunctions||Reflection||RoHS-Conform|
|Absorption||Low||Heating due to absorption: may break at high power||Wide angle tolerance||Level of blocking depends on thickness. Typ. 2 – 3mm||May fluorescence when blocking UV light||Low||Many types “No” but with “legal” exeption status|
|Dielectric||High in small volumes. Low to mid in high volumes||Withstand high optical power due to low absorption||Functional spectrum blue shifts with angle||May be very thin eg 0.5mm||None||Low in transmission region. High in blocking region||Yes|
In addition, there are four basic functional types of filters. These are represented below as symbolic transmission curves as a function of wavelength:
We offer filters cut to any shape. Black anodized aluminium rings may be provided for ease of mounting.
|Specifications||Absorption Filters||Dielectric Filters|
|Range of dimensions||1mm – 300mm||1mm – 300mm|
|Highest available surface figure||λ/4 (DIN: 3/0,5)||λ/4 (DIN: 3/0,5)|
|Highest available surface quality||40-20 S/D (DIN: 5/2×0,16)||40-20 S/D (DIN: 5/2×0,16)|
|Highest available tolerance of beam deviation||3” (arcsec)||3” (arcsec)|
|Blocking||—||OD3 (standard), OD5 (high blocking)|